1. Field of the Invention
The present invention concerns a method for the fabrication of wire anodes for electrolytic capacitors. It also concerns a device enabling the implementation of this method.
2. Description of the Prior Art
Electrolytic capacitors are used above all because of their high capacitance in a small volume. At present, there are three classes of electrolytic capacitors on the market, having a low CxV product. There are aluminium and liquid electrolyte capacitors: the anode is made of aluminium and the cathode is an electrolytic liquid. There are also tantalum and solid electrolyte capacitors: the anode is made of tantalum and the cathode is a solid semiconductor electrolyte. Finally, there are aluminium and solid electrolyte capacitors: the anode is made of aluminium and the cathode is a solid semiconductor electrolyte.
Of late, the last-named class has definitely seen development. The firm Philips has developed several ranges of aluminium and solid electrolyte capacitors. The anode is made with a wound or folded foil. The method of fabrication with a foil anode comprises the following steps:
cutting out the aluminium foil;
etching the foil;
folding the etched aluminium foil in the case of radial capacitors or winding it in the case of axial capacitors;
anodizing the foil to form a thin film of alumina;
forming the solid electrolyte (manganese dioxide) by pyrolysis;
positioning the cathode.
The method used to fabricate these capacitors is quite complicated. It includes a folding operation, part by part, for radial capacitors or a winding operation, part by part, for axial capacitors. The step for the formation of the solid electrolyte is the most difficult one. Several cycles of pyrolysis are needed (four in principle) and these cycles must take place under well-determined conditions of temperature and duration. These are operations that are very difficult to master, for the base product used is a very aggressive solution of manganese nitrate. The conversion into manganese dioxide has to be done very quickly. A post-forming operation is needed to repair the layer damaged by the nitrate.
There is also a method, known from the patent FR 1 496 345, for the fabrication of the anode of an electrolytic capacitor with a wire of an anodizable metal. According to this patent, the metal wire is first of all subjected to a stream of particles of a hard material. One of the ends of the wire thus processed is slipped into a blind hole made in a compression piston. The rest of the wire is then in the cavity of the compression sleeve associated with the punch. With the bottom of the sleeve being blocked, the piston is pushed towards the bottom of the cavity, and this has the effect of compacting the wire. Thus, a block with a large active surface is obtained. To bring the block out of its housing, the bottom of the cavity is opened and the piston continues its forward movement to push out the anode block. This block is detached from the piston and is ready to undergo the operation of forming the oxide layer.
This method has several major drawbacks. It is hard to apply it on an industrial scale because of the use of determined lengths of wire, which are difficult to handle and insert into the hole of the piston. The present trend towards miniaturization makes it imperative to have anodes of small sizes (of the order of some mm.sup.3). This makes it practically impossible to make the device for the implementation of the method, for to have a sufficient active surface of anode, it is necessary to use a metal wire with a diameter of the order of 0.2 mm for a length which may be 500 mm. Under these conditions, it is extremely difficult to make a piston pierced with a blind hole and a sleeve with appropriate dimensions. Finally, the density of pressure is not homogeneous, because of the friction that occurs between the external surface of the block being compressed and the walls of the sleeve. This friction is all the greater as the block being compacted is long and as, until the compacting is completed, it remains slightly elastic.
In order to overcome these drawbacks, the Applicant has conceived an original method enabling the compacting of small-sized blocks of anodes at a high rate. The device made to implement the method according to the invention also enables the resolving of the problems due to the density of pressure.